P RODUCTO GRILLADO CR2-M ET

Masterbatch can be prepared by using high-shear compounding devices, such as TSE or internal mixer. In order to disperse masterbatch effectively by single-screw extruder (SSE), there is a need to have high-shear mixing elements. New Castle Industrial, a SSE manufacturer, has developed a patented technology called NanoMixer. The NanoMixer is reported to product comparable dispersion and property as compared to twin-screw extrusion [8].

The masterbatch technique advantage is that the PP/PPMA compound pre-treated before introducing MMT and thus the introduced shear will promote the incorporation of clay into the polymer matrix. Conversely, in direct compounding, the introduced shear in the screw forces the clay layers to an unfavourable energy situation [9].

2.6 Processing of Polypropylene Nanocomposite

Most rheological studies show that the PPNCs melt flow is very comparable to the pure PP material. This fact is confirmed for the clay content that does not exceed 6% [32]. Thus there will be no extra cost in using PPNCs with existing processing machines. However, the dispersion problems have limited their applications.

During melt process the layered silicate and polymer matrix are mixed together in a molten condition. The polymer molecules, then crawl into the interlayer spacing of the clay particles and forms either an intercalation or an exfoliation systems. Figure 2.7 represents a schematic illustration of PPNCs formation by direct melt intercalation in MMT [28].

Figure 2.7: The intercalation process between a polymer melt and silicate layer A range of PPNCs with structures from intercalated to semi-exfoliated can be obtained, depending on the degree of penetration of the polymer chains into the silicate galleries. The polymer melt flow and the compounding conditions play an important role to have a successful PPNCs processing.

2.6.1 Effect of the base polymer on PPNCs processing

The characteristic properties of PP raw material (mainly MW and molecular weight distribution (MWD)) play an important role in making PPNCs. During twin-screw extrusion, the length of the polymer molecular backbone not only affects intercalation of the clay but also affects the melt viscosity. Melt viscosity directly affects shear stress applied to the polymer melt. Various PP materials have different melt flow rates. For example, the mechanical properties of PP with different melt flow rates produced by masterbatch TSE were examined [8]. At the low melt flow range, good improvement in mechanical properties was reported. However, for high melt flow polypropylene, the improvement in mechanical properties was relatively small. The enhanced properties for low melt flow PP are attributed to the corresponding higher melt viscosity with increased shear deformation of ingredients during the extrusion process. Also, the melt viscosity of low melt flow PP has a better match with the

melt viscosity of masterbatch to enhance clay dispersion. However it has been reported by Kim et al. [20] that in case of using the compatibilizer, high or low viscosity of PP can produce a uniform dispersion. This indicates that PPMA can overcome problems that could be generated from insufficient produced shear in

processing stage of the PP material that has low viscosity.

2.6.2 Effect of compounding conditions on PPNCs formation

Melt extrusion processing conditions are some of the most important factors to disperse layered silicate. It clearly appears that introducing sufficient shear stress is an essential to PPNCs formation. Wang et al. [19] studied the shear effect on the dispersion of the PP. They found that a partially intercalated morphology of PPNCs can be achieved without the help of shear. Combination

with shear, a much better dispersion is obtained. Furlan et al. [33] prepared

PP/MMT without compatibilizer, by using a co-rotating TSE, and showed that the shear intensity is determinant for morphology type, thermal and mechanical properties of PPNCs. They showed that the low and medium shear introduce higher crystallinity than PP (6 and 14 %), but lower crystallization temperature. On the other, the more semi-exfoliated sample as shown by TEM (high shear) increased the crystallization temperature and doesn‟t present a notable change in the crystallinity. However, this study did not show the MMT loading level in PPNCs.

Peltola et al. [34] studied the effect of screw speed on the degree of exfoliation of clay particles by using co-rotating TSE. It was showed through the TEM images that screw speeds can influence the PPNCs structure to be either

Svoboda, suggests that even a highly exfoliated system is obtained, there was no notable change on the mechanical properties of PPNCs.

Another study by Cho et al. [32] compared three routes of mixing of PPNCs. The first route includes preparing of a masterbatch in TSE letdown with SSE. The second route uses a masterbatch in TSE letdown with TSE. The third route includes a direct compounding by using TSE. The results showed that a masterbatch letdown with SSE was better dispersion and mechanical properties than the direct compounding in TSE. Very comparable results between the masterbatch process with SSE and TSE are obtained. The two stage master batch processing is used by Treece et al. [35], in which MMT is first melt blended with PPMA before subsequent dilution with polymer matrix. The result shows an improvement in degree of exfoliation and dispersion relative to single stage processing at 3 and 5% loadings.

Recently, Lertwimolnun et al. [36, 37] studied the effect of three screw profiles and processing parameters (feed rate, screw speed and barrel temperatures) on the PPNCs structure by using XRD. The results showed that the level of exfoliation was dependent of the processing parameters. Conversely, intercalation system was quite independent of the processing parameters. It was also found that a partial control of the exfoliation can be obtained by the screw profile with different imposed shear rates and residence time. Quintanilla et al. [38] prepared PPNCs using compatibilizer and twin screw extruder in two-step mixing and one-step mixing. The better dispersion and exfoliation were obtained for the two-step mixing conditions.

Modesti et al. [39, 40] showed that the processing barrel temperature is a very important parameter: using lower temperature produces higher viscosity and

consequently higher shear stress and therefore, the exfoliation of the clay is promoted. They presented that the processing conditions was not strongly influenced the TGA result; instead, it greatly affect the dynamic-mechanical properties. Bettini et al. [41] reported that the processing conditions (screw speed and feed rate) has a minor effect on the PPNCs interaction as compared to the compatibilizer addition. Gopakumar et al. [42] did a batch mixing with a shaft speed of 2000 rpm until the material reached 190°C (90 seconds) and compared the results with extrusion masterbatch method using co-rotating twin screw extruder. Better dispersion was obtained when a batch mixing is used (measured by XRD), resulting in better tensile strength (3-6%), and lower strain at yield (30%) as compared to Extrusion sample. XRD patterns showed no peak indicating very exfoliation system. However, careful should be taken when only XRD results are examined, because it offers little scope for such quantifications.